Presently, 3-phase induction motor drive systems are finding increased use in industry in robot, machine tool and similar type drive applications. Characteristic of such applications are sudden variations in the moment of inertia experienced by the motor shaft, as well as frequent extraneous load disturbances which are transmitted back to the shaft. Such moment of inertia variations and load disturbances, by their lack of predictability, create undesirable motor control problems. The motor's transient response to such disturbances generally results in deviations from the desired motor performance for the particular application. Thus, it is desirable to provide a robust drive system for all such applications, which has predetermined acceleration and deceleration characteristics and which is insensitive to sudden moment of inertia and other parameter variations and to load disturbances of the type that may be encountered in robot and machine tool applications.
One solution known in the art to the problem of overcoming drive system parameter variations and load disturbances is sliding mode control. In general, a motor under sliding mode control is operated in accordance with a predetermined trajectory which is defined on a phase plane coordinate system. In other types of motor control, the response of the control system to a parameter variation is defined in terms of the transfer function for the particular system. The transfer function is derived from the theoretical relationships (or simplified models thereof) which describe motor operation. As a result, parameter variations will directly cause a change in the transfer function and hence in the operation of the motor under control. In sliding mode control, the predetermined trajectory, which is arbitrarily defined by the control system designer, dictates motor operation under the control of the drive system. Since the trajectory is unrelated to any theoretical relationships which describe the operation of the motor, motor operation under sliding mode control is substantially insensitive to parameter variations. A more detailed description of the theory and application of sliding mode control to a variety of systems is found in the text "Control Systems of Variable Structure", by V. Itkis, Wiley, 1976.
In the past, sliding mode control has been used with some success in analog type servo applications. One example of such an application is described in a paper entitled "MOSFET Converter-Fed Position Servo System with Sliding Mode Control" by Harashima, Hashimoto and Kondo, Conference Record of the Power Electronic Specialists Conference, pp. 73-79, 1983. The system disclosed therein is limited in its application to controlling the operation of a DC motor in the sliding mode.
Attempts to apply sliding mode control to AC induction motors have experienced problems in the past. These problems stem primarily from the complexity of the induction motor mathematical model and the corresponding difficulty of applying the sliding mode methodology to that model. "Application of Sliding Modes to the Control of Electrical Drives" by A. Sabanovic, Conference Record IEEE-IAS Annual Meeting, pp. 553-558, 1982. The mathematical model of the three-phase induction machine to which sliding mode control is to be applied is disclosed in that paper as a system of non-linear differential equations of the fifth order. The resulting mathematical model of the sliding mode control scheme for that motor is of such a complexity as to preclude its practical application.
Thus, notwithstanding its desired advantages, the practical and economically feasible application of sliding mode control to the operation of an induction motor has heretofore proved elusive and difficult to implement by those skilled in the art.